A stepper motor has 2 coils. The Unipolar stepper's coils are split in half and the middle wire goes to ground. On some steppers the middle wires are separate (6 wires) and on others they are connected inside the stepper (5 wires). Voltage goes only in one direction through the half coils.

In the Bipolar stepper, the coils are not split and the voltage goes both ways through the coils. You will need a H-bridge to drive the coils properly, one H-brdidge for each coil (think as 2 DC motors in one). L293D or L298 dual H-bridge will work with this type of steppers (one for each stepper).

The bipolar stepper has a better power/weight ratio (less wire on the coils).